XML 57 R46.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Hedge and private equity funds included in other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | $ 151.6 | $ 125.3
Equity method eligible private equity securities, at fair value |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 126.4 | 83.2
Forward Contracts |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 0.0 | (3.7)
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 11.7 | 4.0
White Mountains |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 289.7 | 208.8
White Mountains | Equity method eligible private equity securities, at fair value |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | $ 97.3 | $ 58.0

X
- Definition Amount of investments classified as other. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(f,g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_OtherInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=wtm_HedgeFundsAndPrivateEquityFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=wtm_LimitedliabilitycompaniesandprivateequitysecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_ForwardContractsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_OtherLongTermInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_WhiteMountainsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |